Date of this Version
MwRSF Research Report No. TRP-03-251-11
Provisions for the design of roadside foreslopes are not readily available. As a result, engineering judgment is often employed. Unfortunately, this can lead to inconsistent designs, where, inevitably, some designs will be too costly and other designs will be too dangerous. Therefore guidance has been compiled to lend consistency to the design of these foreslopes while maintaining the most economical and safe design.
This guidance was prepared after conducting a benefit-cost analysis using the Roadside Safety Analysis Program (RSAP). A large test matrix was developed in an attempt to simulate the most possible scenarios, leaving interpolation to a minimum. However, before the analysis could be run, the severity indices associated with foreslopes needed to be updated to accurately reflect vehicle damage and injury levels caused during an encroachment occurring at an average impact speed. Current indices are overestimated because they were based on a survey given out to highway safety officials who were most likely biased toward high-speed accidents. More data is being collected and will be added to the results of this report in phase two.
To update the severity indices, accident data from the State of Ohio was analyzed using a program called Global Mapper, which allowed the user to measure topographical features, such as foreslopes, heights, and offsets. A method to account for underreported accidents on flat slopes is presented as well. Finally, equations for determining accident cost as a function of the traffic volume are given in conjunction with examples that demonstrate the use of these equations.